Promotion of NH3-SCR activity by sulfate-modification over mesoporous Fe doped CeO2 catalyst: Structure and mechanism

The mesoporous Fe doped CeO2 catalyst after modifying organic sulfate functional groups show an excellent activity with above 80% NOx conversion in a temperature range of 250-450 degrees C. These organic-like sulfate groups bound to the Fe-O-Ce species leads to the strong electron interaction between Fe3+-O-Ce4+ species and sulfate groups, which modifies the acidity and redox properties of catalyst. The strong ability of (SO)-O-=/S-O in sulfate groups to accommodate electrons from a basic molecule is a driving force in the generation of acidic properties, and thus promotes to produce new Bronsted acid sites. The bondage of Fe-O-Ce species obviously inhibits the creation of thermostable bidentate NO3- species. Besides, the redox cycles between Fe3+ and Ce4+ are disrupted, thus inhibiting NH3 oxidation at medium-high temperatures and resulting in the increase of NOx conversion. Furthermore, the in situ DRIFTS results show that for the fresh samples, the coordinate NH3 reacts not only with NO3 through L-H mechanism, but also with oxygen species to form NOx. Differently for sulfated sample, the coordinate NH3 might react with achieved NO2 instead of the oxygen species through E-R mechanism, meanwhile the NH4+ could react with the NO3- species through L-H mechanism.